This invention relates to a process for preparing urea from ammonia and carbon dioxide.
If ammonia and carbon dioxide are fed into a synthesis zone under a suitable pressure (for instance 125-350 atm) and at a suitable temperature (for instance 170.degree.-250.degree. C.), first ammonium carbamate is formed in accordance with the reaction: EQU 2 NH.sub.3 +CO.sub.2 .fwdarw.H.sub.2 N--CO--ONH.sub.4
The resulting ammonium carbamate is subsequently converted to urea by dehydration according to the reaction: EQU H.sub.2 N--CO--ONH.sub.4 .revreaction.H.sub.2 N--CO--NH.sub.2 +H.sub.2 O
The degree to which this conversion into urea proceeds depends, among other things, upon the temperature and amount of excess ammonia that is present in the synthesis zone. The resulting reaction product formed by this process is an aqueous solution substantially consisting of urea, water, ammonium carbonate, and uncoverted ammonia. This ammonium carbamate and the unconverted ammonia must be removed from the urea product solution, and they are most often recycled to the synthesis zone.
The synthesis zone may consist of separate zones wherein ammonium carbamate is primarily formed in the first zone and this ammonium carbamate is converted into urea primarily in the second zone. These zones, however, may also be combined in a single piece of apparatus.
One process for the preparation of urea which has found wide use in practical application is described in European Chemical News, Urea Supplement of Jan. 17, 1969, at pages 17-20. In the process there disclosed, the urea synthesis solution is formed in a synthesis zone maintained at a high pressure and temperature, and is thereafter subjected to a stripping treatment at synthesis pressure by heating the solution and contacting it countercurrently with a gaseous carbon dioxide stripping gas, so as to decompose a major part of the carbamate present therein. The gas mixture thus formed, containing ammonia and carbon dioxide, together with a small amount of water vapor and the carbon dioxide used in the stripping treatment, is removed from the remaining product stream and introduced into a condensation zone wherein it is condensed to form an aqueous ammonium carbamate solution. This aqueous carbamate solution, as well as the remaining non-condensed gas mixture, is recycled to the reaction zone for conversion to urea. The condensation of this gas mixture returned to the reaction zone provides the heat required for the conversion of ammonium carbamate into urea, and no heat need be supplied to the reaction zone from the outside. In addition to using carbon dioxide as the stripping gas as described in this publication, the stripping can also be carried out with gaseous ammonia, an inert gas or with a mixture of at least two of these gases.
The heat required for the stripping treatment is provided by the condensation of high pressure stream of 15 to 25 bar on the shell side of the tubes of the vertical heat exchanger in which the stripping is effected. The gas mixture obtained from this stripping treatment is mostly condensed in a first condensation zone wherein it is absorbed in an aqueous solution obtained from the further treatment of the urea-containing solution downstream. The aqueous ammonium carbamate solution thus formed, together with the remaining non-condensed gas mixture, are introduced into the synthesis zone for the formation of urea. In this synthesis zone, the heat required for the conversion of ammonium carbonate into urea is obtained by further condensation of this gas mixture to ammonium carbamate.
The stripped urea synthesis solution is subsequently expanded to a low pressure of, for instance, 2 to 6 bar, and introduced into a decomposition zone where it is heated by means of steam in order to remove a further amount of ammonia and carbon dioxide still remaining in the stripped urea solution in the form of ammonium carbamate. The resulting gas mixture, which also contains water vapor, is introduced into a second condensation zone wherein it is condensed and absorbed at a low pressure in an aqueous solution, and the dilute carbamate solution thus formed is pumped back to the high pressure section of the urea synthesis and eventually introduced into the synthesis zone. The remaining urea-containing product stream leaving this decomposition zone is subjected to further expansion and is concentrated to form a urea solution or melt that may be further processed to form solid product urea. To this end, the aqueous urea solution is evaporated, usually in two evaporation stages, and the resulting urea melt is processed to form granules, or the urea solution is crystallized. The gases obtained in the evaporation or crystallization, which contain in addition to water vapor an amount of ammonia, carbon dioxide, and entrained fine droplets of urea, are condensed to form process condensate. A part of this process condensate is used as absorbent for the gas mixture condensed in the second condensation zone. The remaining part can be treated with high pressure steam to hydrolyze the urea contained therein and to remove and recover the ammonia and carbon dioxide decomposition products, along with the ammonia and carbon dioxide that was already present.
It is known to incorporate into such a process an additional decomposition step in which further amounts of ammonium carbamate still present in the stripped urea synthesis solution are decomposed at an intermediate pressure of 12 to 25 kg/cm.sup.2. In U.S. Pat. No. 4,354,040, the heat released upon condensation of such a gas mixture obtained in this additional decomposition step is transferred to the evaporation zone by means of passing a urea crystal suspension through the condensation zone via cooling tubes. In this manner, the heat from the condensation of this gas mixture is transferred to the crystal suspension to provide heat for the evaporation of water in the evaporation zone. However, the pumping and circulation of a crystal suspension through cooling tubes has several undesirable effects, including that the presence of solid particles gives rise to disturbances in the process operation, and erosion of the cooling tubes may occur. Moreover, in the process there described the heat absorbed by the crystal suspension is only partially utilized in the crystallization process, inasmuch as a part of the heated crystal containing solution is carried off as product.
It has furthermore been proposed to use the heat released in the condensation of such an intermediate pressure gas mixture containing ammonia and carbon dioxide for the concentration of a urea solution (see U.S. Pat. No. 3,366,682). This further known process does not involve a stripping treatment, but expands and heats the urea synthesis solution in two pressure stages, and the gases released in the expansion are separated from the respective remaining urea-containing solutions. The solution obtained in the first pressure stage, after separating off the gas mixture released upon expansion, is heated and the further gas mixture thus formed is passed in heat exchange with the urea solution to be evaporated, whereupon this solution is concentrated. The gas mixture is cooled as a result of this heat exchange, but is only partially condensed.
It has now been found that for the evaporation of urea solutions, a more economic utilization can be made of the heat released in the condensation of ammonia and carbon dioxide containing gas mixtures obtained in the urea preparation process at sufficiently high pressure levels, if the dew point of the gas mixture to be condensed is raised such that the gas mixture condenses virtually completely during the heat exchange. In this manner, not only is the sensible heat available, but also the heat of dissolution and the heat of condensation can be utilized almost completely in heat exchange with the evaporation stage.